Crustal Velocity Structure across the Southern Korean Peninsula from Earthquake Traveltime Inversion

Seismic traveltime inversion methods have been developed to estimate seismic velocity structures by mainly using body-wave data and more particularly active-source data. We applied the method to data from a natural earthquake to determine 2-D crustal velocity structures for the southern Korean Peninsula. Recent growth of seismograph networks in the region enables us to apply the method to arrival times of regional seismic phases from moderate-to-large earthquakes. Velocity and acceleration data of the M _W 5.6 Shimane, Japan, earthquake on 8th April 2018 recorded on seismograph networks in the Korean Peninsula were tested. Seismic traveltime inversion was conducted based on a model parameterization and a ray tracing method. After P- and S-wave arrival times were manually picked, several 2-D profiles were estimated for back-azimuthally selected paths crossing the major tectonic structures in the Korean Peninsula aligned from the epicenter by collecting nearby stations. With constraing velocities in ranges 6.0-8.0 km/s for P-wave and 3.5-4.5 km/s for S-wave, velocities and layer thicknesses were inverted by using a damped least-squares technique updating, both velocities and boundary depths simultaneously. The differences in traveltime for each propagation resolve variations in crustal thickness. In particular, arrival times of head waves propagating along the mantle lid showed a drastic change with epicentral distance. Results from the traveltime inversion are interpreted as that the changes are originated from the layering of the mantle lid.